Linear copolyesters have been produced heretofore for various purposes, particularly for the production of films and fibers, but the known polymers of this type have not been as effective as would be desired for certain applications. In particular, certain polymers of this type did not possess superior tear strength, tensile strength, flex life and abrasion resistance which would be required for many uses such as hydraulic hose and cable coverings. Recently, a copolyetherester has been discovered which possesses such properties. Although this copolyetherester is significantly superior to those previously known in the art in many regards, it has presented a problem with regard to resistance to oxidative degradation. That is to say, with prolonged exposure to temperatures above about 120.degree. C. the copolyetherester has tended to degrade. For applications such as those in the hydraulic hose and wire insulation fields where temperatures above 120.degree. C. up to about 180.degree. C. are often encountered, an improvement in the thermostability of the copolyetherester is necessary to avoid this problem of oxidative degradation.